This invention relates to an apparatus for forming the actual value of a torque in a three-phase synchronous machine with current-block feeding, i.e., in a machine having windings fed with a-c current in the form of a positive rectangular wavefrom with pulses interleaved timewise with pulses of a negative rectangular waveform. The three a-c feeding currents are mutually staggered by 120.degree. of an operating cycle of the synchronous machine, while respective gaps of 60.degree. exist between the alternating positive and negative current blocks or pulses of each current signal. In such a system, the rise and fall of each current block, i.e., the leading and trailing edges of the current pulses, always take place within a predetermined finite time period.
Synchronous machines with trapezoidal electromotive force (EMF) characteristics generate a constant torque if the machines are fed with a-c currents having rectangular pulses. Since the voltages and currents contributing to the formation of power of all three phases are equal, the resulting actual torque value is proportional to the feeding or driving current. Because the current for each phase is zero during two 60.degree. intervals of each machine operating cycle, the measurement of the current requires measurement and detection in at least two phases. Although the measurement of a feeding current in only two phases calls for but a small amount of hardware, a technical problem arises from the fact that no exactly rectangular current pulses exist in practice. Instead, there is a delay time (a finite interval) for the rise and fall of each current pulse. This delay must be taken into consideration in the generation of actual torque values, inasmuch as a measurement error occurs if with switching-off the block current in a first or second phase, the block current of the second or first phase, respectively, is evaluated uncorrected because the rectangular pulses of the feeding currents have a delay in their rise times so that too small a current value is measured.
An object of the present invention is to provide an improved apparatus for generating the value of a torque in a three-phase synchronous machine.
A more particular feature of the present invention is to provide such an apparatus which generates an actual torque value by measuring the feeding current in two phases.